US President Donald Trump signed a revised ban on travellers from some Muslim-majority nations Monday - one with a reduced scope so Iraqis and permanent US residents are exempt.

After his first wide-ranging and controversial restrictions were slapped down by the federal courts, Trump signed an order freezing new visas for Syrians, Iranians, Libyans, Somalis, Yemenis and Sudanese citizens.

The order did not affect pre-existing visas, the White House said. The measure is due to come into effect on 16 March.

Jeff Sessions, the attorney general, said the order "responsibly provides a needed pause so we can carefully review how we scrutinise people coming here from these countries of concern".

"Three of these nations are state sponsors of terrorism," Sessions added, referring to Iran, Sudan and Syria, adding that others had served as "safe havens" for terror operatives.

The US secretary of state, Rex Tillerson, said the renewed ban on travellers was "a vital measure for strengthening our national security".

"With this order, President Trump is exercising his rightful authority to keep our people safe," he said.

However, human rights groups said the order was still a ban on Muslims and would be challenged in court.

Omar Jadwat, of the American Civil Liberties Union, said: "The Trump administration has conceded that its original Muslim ban was indefensible. Unfortunately, it has replaced it with a scaled-back version that shares the same fatal flaws.

"The only way to actually fix the Muslim ban is not to have a Muslim ban.

"Instead, President Trump has recommitted himself to religious discrimination, and he can expect continued disapproval from both the courts and the people.

"What's more, the changes the Trump administration has made, and everything we've learned since the original ban rolled out, completely undermine the bogus national security justifications the president has tried to hide behind and only strengthen the case against his unconstitutional executive orders."

Senate Democratic leader Chuck Schumer said the measure should be repealed, adding: "A watered down ban is still a ban."

However, Republican Senator Lindsey Graham praised the ban and said he expected it would survive scrutiny by the courts.

"I believe the new order will withstand legal challenges as it's drafted in a fashion as to not be a religious ban, but a ban on individuals coming from compromised governments and failed states. This executive order will help achieve President Trump’s goal of making us safer," Graham, who has criticized some of Trump's policies, said in a statement.

Iraq's new vetting

Iraq was taken off the banned list because the Iraqi government has imposed new vetting procedures, such as heightened visa screening and data sharing, and because of its work with the United States in countering Islamic State militants, a senior White House official said.

Thousands of Iraqis have fought alongside US troops for years or worked as translators since the US-led invasion in 2003. Many have resettled in the United States after being threatened for working with US troops.

Trump's first order was seen by opponents as discrimination against Muslims but the White House official said the new order was based on national security concerns and had nothing to do with religion.

"It is substantially different from the first order yet it will do the same thing in this important way: It will protect the country and keep us safe," the official said. The administration would reset the clock on the 90-day travel ban.

The White House official said the new executive order also ensures that tens of thousands of legal permanent residents in the United States - or green card holders - from the listed countries would not be affected by the travel ban.

Trump's original travel ban resulted in more than two dozen lawsuits in US courts. The state of Washington succeeded in having it suspended by the 9th Circuit Court of Appeals by arguing that it violated constitutional protections against religious discrimination.

The original order barred travellers from the seven nations from entering for 90 days and all refugees for 120 days. Refugees from Syria were to be banned indefinitely but under the new order they are not given separate treatment.

No 'chaos' at airports

Trump publicly criticised judges who ruled against him and vowed to fight the case in the Supreme Court, but then decided to draw up a new order with changes aimed at making it easier to defend in the courts.

Refugees who are "in transit" and already have been approved would be able to travel to the United States.

"There’s going to be a very orderly process," a senior official from the Department of Homeland Security said. "You should not see any chaos so to speak, or alleged chaos at airports. There aren’t going to be folks stopped tonight from coming into the country because of this executive order."

The FBI is investigating 300 people admitted into the United States as refugees as part of 1,000 counter-terrorism probes involving Islamic State or individuals inspired by the militant group, congressional sources told Reuters on Monday, citing senior administration officials.

An FBI spokeswoman said the agency was consulting its data to confirm the information.